1. Field of the Invention
The present invention relates to silver-colored pigments comprising nonmetallic platelet-shaped synthetic substrates and at least one ilmenite-containing coating with an amount of iron compounds, calculated as elemental iron, in the pigment of less than 5% by weight, based on the total weight of the pigment, a process for the preparation thereof as well as the use thereof. The invention further relates to an object which is provided with the silver-colored pigments according to the invention as well as a preparation which comprises the latter.
2. Description of Related Art
WO 2004/099319 A2 describes interference pigments with a high covering power, high luster and color change at varying viewing angles, comprising a platelet-shaped substrate and at least one FeTiO3-containing layer. The proportion of FeTiO3 in the layer is 8 to 100% by weight, based on the total weight of the layer. These interference pigments are prepared by the simultaneous deposition of titanium(IV) oxide hydrate and iron(III) oxide hydrate on the substrate surface and subsequent thermal treatment under reducing conditions. A homogeneous distribution of ilmenite inside the coating is thus to be guaranteed.
WO 97/43348 A1 describes unsupported single or multi-coat pearlescent pigments consisting of iron titanate and possibly titanium oxide and/or iron oxide. To prepare these pearlescent pigments, titanium dioxide platelets obtained via a belt process are coated, without intermediate drying, with iron oxide in a wet process, and the resulting pigments are dried and calcined in an oxidizing or reducing gas atmosphere. The pearlescent pigments obtained have a high color strength and exhibit for example a color flop from red to gold or from gold to red. It is also possible to obtain blue-black lustrous pigments.
EP 0 246 523 A2 relates to colored pearlescent pigments, which are for example black-gold, black-red, black-green or dark blue, based on a platelet-shaped substrate which is provided with a compact coating containing iron(II) oxide. The layer containing iron(II) oxide can have a different composition here depending on the preparation process and the substrate used. This invention is aimed at pigments with a high conductivity, which manifests itself in a good shielding from electromagnetic interference fields.
Furthermore, these pigments can be easily aligned in magnetic fields because of their magnetizability.
Commercial ilmenite-coated pearlescent pigments based on natural mica are characterized by R. Maisch in the article, New effect pigments from grey to black, Progress in Organic Coatings, 22 (1993) 261-272.
The use of ilmenite-containing interference pigments, which can be black for example and exhibit strong interference colors at the specular angle, to produce forgeryproof documents and packaging is known from EP 0 681 009 B1.
Many everyday fields can no longer be imagined without silver hues, silver vehicles lend distinction to the streetscape and a silver coloring or varnish gives everyday objects like coffee machines, television sets or packaging a high-class appearance. Metallic effect pigments, in particular aluminum effect pigments, critically determine the silver hue. Regulatory limitations, such as e.g. in the color cosmetics sector, can however limit the field of use of metallic effect pigments. If electromagnetic shielding has a role to play, such as for example in the case of cellular phones, metallic effect pigments are often dispensed with in the varnish, although e.g. aluminum effect pigments, contrary to widespread opinion, are not electrically conductive. If the metallic effect and the visual impression of an aluminum effect pigment are to be preserved in an application, a simple substitution by a silver-colored pearlescent pigment is not possible. Commercial silver-colored pearlescent pigments for example generally do not possess the neutral silver hue that is characteristic of aluminum effect pigments. There is a need, therefore, for pigments that in terms of the optical properties such as, for example, silver hue, opacity, metallic gloss or light/dark flop, come close to metallic effect pigments, yet contain no metal and can thus contribute to the field of use of metallic effect pigments.
The problem on which the present invention was based was that of providing highly lustrous silver-colored pigments which in terms of their visual impression have properties that are characteristic of metallic effect pigments. The silver-colored pigments are not to differ, or to differ only insignificantly, from commercial aluminum effect pigments in their appearance. At the same time, the silver-colored pigments are to be characterized by a high chemicals stability and temperature resistance. The problem on which the invention is based is further that of providing a process for preparing these silver-colored pigments.